225 research outputs found
Measuring 14 elemental abundances with R=1,800 LAMOST spectra
The LAMOST survey has acquired low-resolution spectra (R=1,800) for 5 million
stars across the Milky Way, far more than any current stellar survey at a
corresponding or higher spectral resolution. It is often assumed that only very
few elemental abundances can be measured from such low-resolution spectra,
limiting their utility for Galactic archaeology studies. However, Ting et al.
(2017) used ab initio models to argue that low-resolution spectra should enable
precision measurements of many elemental abundances, at least in theory. Here
we verify this claim in practice by measuring the relative abundances of 14
elements from LAMOST spectra with a precision of 0.1 dex for objects
with > 30 (per pixel). We employ a spectral modeling
method in which a data-driven model is combined with priors that the model
gradient spectra should resemble ab initio spectral models. This approach
assures that the data-driven abundance determinations draw on physically
sensible features in the spectrum in their predictions and do not just exploit
astrophysical correlations among abundances. Our analysis is constrained to the
number of elemental abundances measured in the APOGEE survey, which is the
source of the training labels. Obtaining high quality/resolution spectra for a
subset of LAMOST stars to measure more elemental abundances as training labels
and then applying this method to the full LAMOST catalog will provide a sample
with more than 20 elemental abundances that is an order of magnitude larger
than current high-resolution surveys, substantially increasing the sample size
for Galactic archaeology.Comment: 6 pages, 3 figures, ApJ (Accepted for publication- 2017 October 9
How Dense of a Circumstellar Medium Is Sufficient to Choke a Jet?
The progenitor stars of stripped-envelope high-velocity supernovae (Ic-BL SNe) can explode inside a dense circumstellar medium (CSM) that extends out to many times the progenitor radius. This complicates the question of whether all Ic-BL SNe harbor a jet, which can tunnel through the star and be viewed on-axis as a long-duration gamma-ray burst (GRB). More specifically, a sufficiently dense CSM might "choke" the jet, redistributing its energy quasi-spherically. In this study, we numerically calculate the CSM density necessary for jet choking. For typical GRBs, we determine the jet is not choked in the CSM unless ρr² > 4×10¹⁹g cm⁻¹; this requires several solar masses of CSM to be situated within 10¹³ cm of the progenitor, a much higher density than any CSM observed. We conclude that typical GRB jets are not choked in the CSM. However, in many cases the CSM has sufficient mass to decelerate the jet to a modest Lorentz factor (Γ ~ 10), which should lead to a long coasting phase for the jet, observable as a long plateau (potentially up to a few days) in the afterglow light curve. For extreme cases of low-energy GRBs in a high-mass CSM, the jet will decelerate to nonrelativistic velocities, causing it to spread modestly to a larger opening angle (θ_j ≈ 20°) before breaking out of the CSM. Even in these extreme examples, the jet does not have time to redistribute its energy quasi-spherically in the CSM before breakout
Label Transfer from APOGEE to LAMOST: Precise Stellar Parameters for 450,000 LAMOST Giants
In this era of large-scale stellar spectroscopic surveys, measurements of
stellar attributes ("labels," i.e. parameters and abundances) must be made
precise and consistent across surveys. Here, we demonstrate that this can be
achieved by a data-driven approach to spectral modeling. With The Cannon, we
transfer information from the APOGEE survey to determine precise Teff, log g,
[Fe/H], and [/M] from the spectra of 450,000 LAMOST giants. The Cannon
fits a predictive model for LAMOST spectra using 9952 stars observed in common
between the two surveys, taking five labels from APOGEE DR12 as ground truth:
Teff, log g, [Fe/H], [\alpha/M], and K-band extinction . The model is then
used to infer Teff, log g, [Fe/H], and [/M] for 454,180 giants, 20% of
the LAMOST DR2 stellar sample. These are the first [/M] values for the
full set of LAMOST giants, and the largest catalog of [/M] for giant
stars to date. Furthermore, these labels are by construction on the APOGEE
label scale; for spectra with S/N > 50, cross-validation of the model yields
typical uncertainties of 70K in Teff, 0.1 in log g, 0.1 in [Fe/H], and 0.04 in
[/M], values comparable to the broadly stated, conservative APOGEE DR12
uncertainties. Thus, by using "label transfer" to tie low-resolution (LAMOST R
1800) spectra to the label scale of a much higher-resolution (APOGEE R
22,500) survey, we substantially reduce the inconsistencies between
labels measured by the individual survey pipelines. This demonstrates that
label transfer with The Cannon can successfully bring different surveys onto
the same physical scale.Comment: 27 pages, 14 figures. Accepted by ApJ on 16 Dec 2016, implementing
suggestions from the referee reports. Associated code available at
https://github.com/annayqho/TheCanno
Distributed Caching for Complex Querying of Raw Arrays
As applications continue to generate multi-dimensional data at exponentially
increasing rates, fast analytics to extract meaningful results is becoming
extremely important. The database community has developed array databases that
alleviate this problem through a series of techniques. In-situ mechanisms
provide direct access to raw data in the original format---without loading and
partitioning. Parallel processing scales to the largest datasets. In-memory
caching reduces latency when the same data are accessed across a workload of
queries. However, we are not aware of any work on distributed caching of
multi-dimensional raw arrays. In this paper, we introduce a distributed
framework for cost-based caching of multi-dimensional arrays in native format.
Given a set of files that contain portions of an array and an online query
workload, the framework computes an effective caching plan in two stages.
First, the plan identifies the cells to be cached locally from each of the
input files by continuously refining an evolving R-tree index. In the second
stage, an optimal assignment of cells to nodes that collocates dependent cells
in order to minimize the overall data transfer is determined. We design cache
eviction and placement heuristic algorithms that consider the historical query
workload. A thorough experimental evaluation over two real datasets in three
file formats confirms the superiority -- by as much as two orders of magnitude
-- of the proposed framework over existing techniques in terms of cache
overhead and workload execution time
Masses and Ages for 230,000 LAMOST Giants, via Their Carbon and Nitrogen Abundances
We measure carbon and nitrogen abundances to a precision of ≾0.1 dex for 450,000 giant stars from their low-resolution (R ~ 1800) LAMOST DR2 survey spectra. We use these [C/M] and [N/M] measurements, together with empirical relations based on the APOKASC sample, to infer stellar masses and implied ages for 230,000 of these objects to 0.08 dex and 0.2 dex respectively. We use The Cannon, a data-driven approach to spectral modeling, to construct a predictive model for LAMOST spectra. Our reference set comprises 8125 stars observed in common between the APOGEE and LAMOST surveys, taking seven APOGEE DR12 labels (parameters) as ground truth: T_(eff), log g, [M/H], [α/M], [C/M], [N/M], and A_k. We add seven colors to the Cannon model, based on the g, r, i, J, H, K, W1, W2 magnitudes from APASS, 2MASS, and WISE, which improves our constraints on T_(eff) and log g by up to 20% and on A_k by up to 70%. Cross-validation of the model demonstrates that, for high-S/N objects, our inferred labels agree with the APOGEE values to within 50 K in temperature, 0.04 mag in A_k, and <0.1 dex in log g, [M/H], [C/M], [N/M], and [α/M]. We apply the model to 450,000 giants in LAMOST DR2 that have not been observed by APOGEE. This demonstrates that precise individual abundances can be measured from low-resolution spectra and represents the largest catalog to date of homogeneous stellar [C/M], [N/M], masses, and ages. As a result, we greatly increase the number and sky coverage of stars with mass and age estimates
Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity
Chemical tagging promises to use detailed abundance measurements to identify
spatially separated stars that were in fact born together (in the same
molecular cloud), long ago. This idea has not yielded much practical success,
presumably because of the noise and incompleteness in chemical-abundance
measurements. We have succeeded in substantially improving spectroscopic
measurements with The Cannon, which has now delivered 15 individual abundances
for ~100,000 stars observed as part of the APOGEE spectroscopic survey, with
precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking
at clusters in abundance space and confirming that they are clustered in phase
space. We identify (by the k-means algorithm) overdensities of stars in the
15-dimensional chemical-abundance space delivered by The Cannon, and plot the
associated stars in phase space. We use only abundance-space information (no
positional information) to identify stellar groups. We find that clusters in
abundance space are indeed clusters in phase space. We recover some known
phase-space clusters and find other interesting structures. This is the
first-ever project to identify phase-space structures at survey-scale by blind
search purely in abundance space; it verifies the precision of the abundance
measurements delivered by The Cannon; the prospects for future data sets appear
very good.Comment: accepted for publication in the Ap
Masses and Ages for 230,000 LAMOST Giants, via Their Carbon and Nitrogen Abundances
We measure carbon and nitrogen abundances to a precision of ≾0.1 dex for 450,000 giant stars from their low-resolution (R ~ 1800) LAMOST DR2 survey spectra. We use these [C/M] and [N/M] measurements, together with empirical relations based on the APOKASC sample, to infer stellar masses and implied ages for 230,000 of these objects to 0.08 dex and 0.2 dex respectively. We use The Cannon, a data-driven approach to spectral modeling, to construct a predictive model for LAMOST spectra. Our reference set comprises 8125 stars observed in common between the APOGEE and LAMOST surveys, taking seven APOGEE DR12 labels (parameters) as ground truth: T_(eff), log g, [M/H], [α/M], [C/M], [N/M], and A_k. We add seven colors to the Cannon model, based on the g, r, i, J, H, K, W1, W2 magnitudes from APASS, 2MASS, and WISE, which improves our constraints on T_(eff) and log g by up to 20% and on A_k by up to 70%. Cross-validation of the model demonstrates that, for high-S/N objects, our inferred labels agree with the APOGEE values to within 50 K in temperature, 0.04 mag in A_k, and <0.1 dex in log g, [M/H], [C/M], [N/M], and [α/M]. We apply the model to 450,000 giants in LAMOST DR2 that have not been observed by APOGEE. This demonstrates that precise individual abundances can be measured from low-resolution spectra and represents the largest catalog to date of homogeneous stellar [C/M], [N/M], masses, and ages. As a result, we greatly increase the number and sky coverage of stars with mass and age estimates
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